This invention relates to a process for the production of polyurethane/urea molded article which exhibits both a high flexural modulus and a high impact strength, from a reaction injection molding process of a reaction mixture in the presence of a catalyst via the one-shot process. The isocyanate component of this invention is a prepolymer of methylenebis(cyclohexyl isocyanate) having an NCO content of 12-27%, an average isocyanate functionality of less than 2.3, a monomer content of greater than about 25% by weight, and containing urethane or urea groups. The isocyanate-reactive blend comprises a polyether polyol or a polyamine having a molecular weight of at least 2000, and a low molecular weight chain extender, wherein the isocyanate-reactive blend has an OH:NH equivalent ratio of from 1:1 to 15:1.
The production of polyurethane moldings via the reaction injection molding (i.e. RIM) technique is well known and described in, for example, U.S. Pat. No. 4,218,543. The RIM process involves a technique of filling the mold by which highly reactive, liquid starting components are injected into the mold within a very short time by means of a high output, high pressure dosing apparatus after they have been mixed in so-called "positively controlled mixing heads".
In the production of polyurethane moldings via the RIM process, the reaction mixture generally comprises an A-side based on polyisocyanates and a B-side based on organic compounds containing isocyanate-reactive hydrogen atoms, in addition to suitable chain extenders, catalysts, blowing agents, and other additives. The polyisocyanates which are suitable for a commercial RIM process are the aromatic isocyanates such as, for example, diphenyl methane-4,4'-diisocyanate (i.e. MDI).
Various patents such as, for example, U.S. Pat. No. 4,937,366, broadly disclose cycloaliphatic isocyanates, including methylenebis(cyclohexyl isocyanate), in a long list of isocyanates which are said to be suitable for use in a RIM process. However, very few of the patents which disclose that cycloaliphatic isocyanates are suitable for use in a RIM process have any working examples wherein a cycloaliphatic isocyanate is used. The RIM examples of U.S. Pat. No. 4,937,366 are all based on a prepolymer of an aromatic isocyanate.
U.S. Pat. No. 4,772,639 describes a process for the production of polyurethane moldings reacting organic polyisocyanates with organic compounds containing isocyanate-reactive hydrogen atoms in the presence of catalysts and auxiliary agents inside a closed mold. The isocyanate component is based on (a1) mixtures of (i) 1-isocyanate-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (IPDI), and (ii) polyisocyanates containing isocyanurate groups prepared by the trimedzation of a portion of the isocyanate groups of 1,6-diisocyanatohexane, or (a2) (i) IPDI and (iii) polyisocyanates containing isocyanurate groups prepared by the trimerization of a portion of the isocyanate groups of a mixture of 1,6-diixocyanatohexane and IPDI. These reaction mixtures are broadly disclosed as being suitable for RIM processing. However, the use of IPDI monomer alone is not disclosed or suggested.
U.S. Pat. No. 4,642,320 discloses a process for the preparation of a molded polymer comprising reacting inside a closed mold a reaction mixture comprising (a) an active hydrogen containing material comprising a primary or secondary amine terminated polyether having an average equivalent weight of at least 500, (b) at least one chain extender, and (c) a (cyclo)aliphatic polyisocyanate, polyisothiocyanate, or mixture thereof, wherein the NCX index is from about 0.6 to 1.5. This process requires that component (a) have at least 25%, and preferably 50% of its active hydrogen atoms present in the form of amine hydrogens. All of the examples disclose a system based on a HDI prepolymer with amine terminated polyethers and diethyltoluenediamine at high mold temperatures and long demold times.
U.S. Pat. No. 4,764,543 discloses aliphatic RIM systems that use very fast reacting aliphatic polyamines. This patent is restricted to total polyurea systems based on chain extenders which are cycloaliphatic diamines and polyethers which are amine-terminated polyethers, with an aliphatically bound polyisocyanate.
RIM systems are also disclosed in U.S. Pat. No. 4,269,945. These systems are based on compositions comprising a polyisocyanate, a hydroxyl-containing polyol, and a specific chain extender. The specific chain extender comprises (1) at least one component selected from the group consisting of (a) a hydroxyl-containing material which is essentially free of aliphatic amine hydrogen atoms, and (b) aromatic amine-containing materials containing at least two aromatic amine hydrogen atoms and are essentially free of aliphatic amine hydrogen atoms; and (2) at least one aliphatic amine-containing material having at least one primary amine group and an average aliphatic amine hydrogen functionality of from about 2 to 16. Both aromatic polyisocyanates and (cyclo)aliphatic polyisocyanates are disclosed as being suitable for this process. All of the working examples in this patent use aromatic isocyanates that may be polymeric in nature.
U.S. Pat. No. 5,260,346 also discloses reaction systems for preparing elastomers via the RIM process. These systems require an allophanate modified polyisocyanate, a hydroxyl group containing polyol, and an aromatic polyamine having at least one of the positions ortho to the amine substituted with a lower alkyl substituent.
U.S. Pat. No. 5,502,147, which is commonly assigned, describes (cyclo)aliphatic isocyanate based RIM systems. These (cyclo)aliphatic isocyanates have a viscosity of less than 20,000 mPa.multidot.s at 25.degree. C., an NCO functionality of 2.3 to 4.0, and are modified by isocyanurate groups, biuret groups, urethane groups, allophanate groups, carbodiimide groups, oxadiazine-trione groups, uretdione groups, and blends thereof. All of the working examples of this application use hexamethylene diisocyanate which is modified by one of the above groups.
U.S. Pat. No. 5,502,150, which is commonly assigned, discloses a RIM process which uses a hexamethylene diisocyanate prepolymer having a functionality of less than 2.3, an NCO content of 5 to 25%, and a monomer content of less than 2% by weight. This prepolymer is reacted with a high molecular weight isocyanate-reactive compound, a chain extender selected from diols and aminoalcohols, and a hydroxyl-based crosslinking compound containing no more than one aliphatic amine hydrogen atom.
Copending application Ser. No. 08/484,402, filed Jun. 7, 1995, which is commonly assigned, discloses a method of producing window gaskets of polyurethane/urea compositions. These polyurethane/urea compositions comprise a (cyclo)aliphatic polyisocyanate having a viscosity of less than 25,000 mPa.multidot.s at 25.degree. C. and an average NCO functionality of about 2.0 to 4.0, with an isocyanate-reactive composition comprising a high molecular weight isocyanate-reactive component and a low molecular weight chain extender, in the presence of a catalyst wherein the reactive components are selected such that the final polyurethane/urea composition has a crosslink density of at least 0.3 moles/kg.
Advantages of the presently claimed invention include the fact that expensive amine terminated polyethers (ATPEs) do not have to be used as the high molecular weight component of the B-side. Also, the prepolymer of this invention does not have to undergo an expensive stripping step, as was required of the prepolymers in U.S. Pat. No. 5,502,150. The low-functionality prepolymers of the present invention also result in excellent impact strengths at high flexural modulus.